Foliiferous wood (i.e., hardwood) is generally easy to impregnate. Solutions containing preservatives can be rapidly forced by means of pressure into the wood. As a rule the entire mass of the foliiferous wood is penetrated by the preservative solution, in contrast to pine wherein only the sap wood is penetrated. It had been earlier thought that this caused few problems since good results had been obtained by the pressure treatment of sleepers of beech with creosote oil.
This opinion has recently been modified. It is clear that foliiferous wood treated with salt-containing preservative solutions provides prolonged periods of use but the preservation treatment has been much less effective than expected. Since the salts are well fixed in the wood, the lessened effect of the treatment has not been considered to depend on the leaching effect of the preservative. The preserving salt has, in all probability, been effective against the wood destroying organisms which appear in and destroy foliiferous wood. The long-term ineffectiveness of the treatment has been observed in several different types of preservatives of different origin. An explanation for the observed long-term ineffectiveness of the preservatives is believed to have been found by electron microscopic studies of the wood. It appears from these studies that the preservatives have not penetrated the cell walls of the wood. The inner part of the cell walls have thus remained untreated. The wood destroying organisms have thus been able to grow within the inner part of the cell walls and destroy the wood.